void main(List<String> args) {
  List goodsList = [
    {"name": "苹果", "price": 10, "count": 100, "selected": true},
    {"name": "香蕉", "price": 5, "count": 200, "selected": false},
    {"name": "西瓜", "price": 100, "count": 5, "selected": true}
  ];
  double value = goodsList.where((element) => element["selected"]).fold(
      0.0,
      (previousValue, element) =>
          previousValue + element["count"] * element["price"]);
  print(value);
}
